[PATCH] timerfd: add TFD_NOTIFY_CLOCK_SET to watch for clock changes

@@ -30,8 +30,13 @@ struct timerfd_ctx {
 	u64 ticks;
 	int expired;
 	int clockid;
+	struct list_head notifiers_list;
 };
 
+/* TFD_NOTIFY_CLOCK_SET timers go here */
+static DEFINE_SPINLOCK(notifiers_lock);
+static LIST_HEAD(notifiers_list);
+
 /*
  * This gets called when the timer event triggers. We set the "expired"
  * flag, but we do not re-arm the timer (in case it's necessary,

@@ -51,6 +56,23 @@ static enum hrtimer_restart timerfd_tmrproc(struct hrtimer *htmr)
 	return HRTIMER_NORESTART;
 }
 
+void timerfd_clock_was_set(clockid_t clockid)
+{
+	struct timerfd_ctx *ctx;
+	unsigned long flags;
+
+	spin_lock(&notifiers_lock);
+	list_for_each_entry(ctx, &notifiers_list, notifiers_list) {
+		spin_lock_irqsave(&ctx->wqh.lock, flags);
+		if (ctx->tmr.base->index == clockid) {
+			ctx->ticks++;
+			wake_up_locked(&ctx->wqh);
+		}
+		spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+	}
+	spin_unlock(&notifiers_lock);
+}
+
 static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
 {
 	ktime_t remaining;

@@ -72,6 +94,12 @@ static void timerfd_setup(struct timerfd_ctx *ctx, int flags,
 	ctx->expired = 0;
 	ctx->ticks = 0;
 	ctx->tintv = timespec_to_ktime(ktmr->it_interval);
+
+	if (flags & TFD_NOTIFY_CLOCK_SET) {
+		list_add(&ctx->notifiers_list, &notifiers_list);
+		return;
+	}
+
 	hrtimer_init(&ctx->tmr, ctx->clockid, htmode);
 	hrtimer_set_expires(&ctx->tmr, texp);
 	ctx->tmr.function = timerfd_tmrproc;

@@ -83,7 +111,12 @@ static int timerfd_release(struct inode *inode, struct file *file)
 {
 	struct timerfd_ctx *ctx = file->private_data;
 
-	hrtimer_cancel(&ctx->tmr);
+	if (!list_empty(&ctx->notifiers_list)) {
+		spin_lock(&notifiers_lock);
+		list_del(&ctx->notifiers_list);
+		spin_unlock(&notifiers_lock);
+	} else
+		hrtimer_cancel(&ctx->tmr);
 	kfree(ctx);
 	return 0;
 }

@@ -113,6 +146,7 @@ static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
 
 	if (count < sizeof(ticks))
 		return -EINVAL;
+
 	spin_lock_irq(&ctx->wqh.lock);
 	if (file->f_flags & O_NONBLOCK)
 		res = -EAGAIN;

@@ -120,7 +154,8 @@ static ssize_t timerfd_read(struct file *file, char __user *buf, size_t count,
 		res = wait_event_interruptible_locked_irq(ctx->wqh, ctx->ticks);
 	if (ctx->ticks) {
 		ticks = ctx->ticks;
-		if (ctx->expired && ctx->tintv.tv64) {
+		if (ctx->expired && ctx->tintv.tv64 &&
+		    list_empty(&ctx->notifiers_list)) {
 			/*
 			 * If tintv.tv64 != 0, this is a periodic timer that
 			 * needs to be re-armed. We avoid doing it in the timer

@@ -218,13 +253,17 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
 	 * it to the new values.
 	 */
 	for (;;) {
+		spin_lock(&notifiers_lock);
 		spin_lock_irq(&ctx->wqh.lock);
 		if (hrtimer_try_to_cancel(&ctx->tmr) >= 0)
 			break;
 		spin_unlock_irq(&ctx->wqh.lock);
+		spin_unlock(&notifiers_lock);
 		cpu_relax();
 	}
 
+	INIT_LIST_HEAD(&ctx->notifiers_list);
+
 	/*
 	 * If the timer is expired and it's periodic, we need to advance it
 	 * because the caller may want to know the previous expiration time.

@@ -243,6 +282,7 @@ SYSCALL_DEFINE4(timerfd_settime, int, ufd, int, flags,
 	timerfd_setup(ctx, flags, &ktmr);
 
 	spin_unlock_irq(&ctx->wqh.lock);
+	spin_unlock(&notifiers_lock);
 	fput(file);
 	if (otmr && copy_to_user(otmr, &kotmr, sizeof(kotmr)))
 		return -EFAULT;

@@ -262,6 +302,14 @@ SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
 	ctx = file->private_data;
 
 	spin_lock_irq(&ctx->wqh.lock);
+	if (!list_empty(&ctx->notifiers_list)) {
+		kotmr.it_value = current_kernel_time();
+		kotmr.it_interval.tv_sec = 0;
+		kotmr.it_interval.tv_nsec = 0;
+		spin_unlock_irq(&ctx->wqh.lock);
+		goto out;
+	}
+
 	if (ctx->expired && ctx->tintv.tv64) {
 		ctx->expired = 0;
 		ctx->ticks +=

@@ -273,6 +321,7 @@ SYSCALL_DEFINE2(timerfd_gettime, int, ufd, struct itimerspec __user *, otmr)
 	spin_unlock_irq(&ctx->wqh.lock);
 	fput(file);
 
+out:
 	return copy_to_user(otmr, &kotmr, sizeof(kotmr)) ? -EFAULT: 0;
 }
 

@@ -247,6 +247,12 @@ static inline ktime_t hrtimer_expires_remaining(const struct hrtimer *timer)
     return ktime_sub(timer->_expires, timer->base->get_time());
 }
 
+#ifdef CONFIG_TIMERFD
+extern void timerfd_clock_was_set(clockid_t clockid);
+#else
+static inline void timerfd_clock_was_set(clockid_t clockid) {}
+#endif
+
 #ifdef CONFIG_HIGH_RES_TIMERS
 struct clock_event_device;
 

@@ -19,6 +19,7 @@
  * shared O_* flags.
  */
 #define TFD_TIMER_ABSTIME (1 << 0)
+#define TFD_NOTIFY_CLOCK_SET (1 << 1)
 #define TFD_CLOEXEC O_CLOEXEC
 #define TFD_NONBLOCK O_NONBLOCK
 

@@ -26,6 +27,6 @@
 /* Flags for timerfd_create.  */
 #define TFD_CREATE_FLAGS TFD_SHARED_FCNTL_FLAGS
 /* Flags for timerfd_settime.  */
-#define TFD_SETTIME_FLAGS TFD_TIMER_ABSTIME
+#define TFD_SETTIME_FLAGS (TFD_TIMER_ABSTIME | TFD_NOTIFY_CLOCK_SET)
 
 #endif /* _LINUX_TIMERFD_H */

@@ -646,6 +646,9 @@ void clock_was_set(void)
 {
 	/* Retrigger the CPU local events everywhere */
 	on_each_cpu(retrigger_next_event, NULL, 1);
+
+	/* Trigger timerfd notifiers */
+	timerfd_clock_was_set(CLOCK_REALTIME);
 }
 
 /*

@@ -658,6 +661,9 @@ void hres_timers_resume(void)
 		  KERN_INFO "hres_timers_resume() called with IRQs enabled!");
 
 	retrigger_next_event(NULL);
+
+	/* Trigger timerfd notifiers */
+	timerfd_clock_was_set(CLOCK_MONOTONIC);
 }
 
 /*